This invention relates generally to cathode ray tube (CRT) automatic tracking circuits that compensate for changes or variations in CRT display characteristics and particularly to an automatic tracking system that is digitally implemented.
Digital television receivers cannot use most conventional analog signal processing circuits. The ready availability of powerful microprocessors and low cost memories enables a significant amount of digital signal processing, however. In an NTSC discrete pixel CRT display, there are approximately 910 pixels per horizontal scanning line of the CRT screen. With suitable digital processing, exact control of each pixel may be obtained.
It is conventional in the art to provide automatic tracking circuits to compensate for CRT "aging." For example, the cutoff characteristic of the CRT may change or its color balance, that is the operational relationship between the red, green and blue electron gun structures may vary and lead to erroneous or poor black and white tracking. In digital television systems, it is known to artificially generate test signals for sampling the pertinent CRT characteristics. The test signals are multiplexed and applied during the vertical blanking interval (VBI) of the television signal to monitor the CRT display characteristics and to enable it to be compensated for deviations from a norm or set of reference values that have been established at the factory. In the ITT publication--DIGIT 2000 VLSI Digital Television System--a complete digitized television receiver system is disclosed. Digital television receivers operating in accordance with that publication are in the prior art. One such television receiver is identified as the Zenith Electronics Corporation, Model No. SE3191H. That television receiver (and the receiver of the publication) applies a series of test signals to a CRT during the vertical blanking interval for determining certain display characteristics. The signals are applied and the resulting data developed in a time multiplexed manner. These signals represent the individual red (R), green (G) and blue (B) cathode currents developed in response to an applied white level test signal and the individual color currents that are developed in response to an applied black level test signal. The results of the test signals are detected and compared with stored reference levels, or values, for those signals. Deviations between the measured data and the reference data are compensated by appropriately changing the operating characteristics of the R, G, and B amplifiers in the television receiver.
In a conventional direct view television receiver, the display area of the CRT is overscanned so that the horizontal lines at the beginning and the end of the VBI are not visible. If the display were vertically displaced to show these horizontal lines, the applied test signals would appear as a multicolored group of lines. In a rear projection television receiver, masking of the display area can be used to hide the test signal lines. In a front projection system, however, masking of the lines caused by the test signals is not readily feasible, and the lines are often annoying to a viewer.
As mentioned, the test signals are multiplexed and applied over a number of sequential fields of the television signal. While the actual number of fields is not critical, in the preferred embodiment of the invention, four successive fields are used. During the first video field, ambient light and CRT leakage signals are sampled; in the next field, the red cathode currents produced in response to the white and black level test signals are sensed; in the following field, the green cathode currents produced in response to the white and black level test signals are sensed; and in the fourth field, the blue cathode currents produced in response to the white and black level test signals are sensed.
In prior art auto tracking systems, the rate of correction is fixed and represents a compromise between rapid correction of the CRT display characteristics and slow correction. Making corrections too rapidly is disturbing in that color and brightness changes will be clearly visible to the viewer. Very slow changes are objectionable in that unbalanced or erroneous signal conditions can remain uncorrected for excessively long periods of time.
With the invention, the velocity or speed of correction is controllable by the manufacturer or designer. This is preferably accomplished in software. Different types of cathode ray tubes can thus be compensated for display characteristic variations at different rates of speed. For example, if a CRT type that is known to be very stable is used, correction may be performed at a slow rate or the correction circuitry may even be disabled. On the other hand, for front projection receivers, it may be desirable to initially correct at a high rate of speed for a few minutes and then to disable the correction circuitry to quickly remove the visible test signal pattern. As will be seen, the inventive system further provides for desensitizing the error signal to minimize small corrections by increasing the amount of error required to initiate a correction sequence.